It has long been known in the packaging industry to employ packaging materials comprising outer layers of thermoplastic, these outer layers being capable of being sealed together by fusion after heating and compression for providing tight and durable sealing joints or seams. For example, such a material may be fed in web form through a filling machine and formed into a tube which is longitudinally sealed by means of a lap joint. This material tube is filled with its contemplated contents, whereafter the tube is transversely sealed and severed to form individual packages which, in a later stage, are finally formed, for example into parallelepipedic packages or cartons.
A packaging material including a metal foil, preferably aluminium foil, is ideally employed when the transverse seals are made by so-called induction sealing. An apparatus for such induction sealing is disclosed and described in Swedish Patent Specification SE 451 973. In this instance, an induction current is induced in the aluminium layer of the packaging material, this current heating the thermoplastic layer disposed adjacent the aluminium layer so that the thermoplastic layer can be sealed to a corresponding thermoplastic layer.
The above-described method of transversely sealing packaging blanks as described in the above-disclosed patent specification has, however, proved to be unsuitable for highly viscous products and products containing particles such as kernels, fruit seeds or fibres. According to the above-described method, two substantially parallel transverse seals are made between which the packaging tube is severed to form individual packaging blanks. In this instance, sealing flaps or fins are created which lie outside the transverse seals and where product residues may accumulate. These product residues can, on storage of the package, take on a highly disagreeable and unhealthy appearance.